Huntington's disease (HD) is a hereditary neurodegenerative disease characterized by selective basal ganglia damage and movement disorders. The genetic defect responsible for HD is an expanded poly-glutamine repeat in the huntingtin protein in excess of that found in unaffected individuals. The mechanism by which the expanded poly-glutamine repeats in huntingtin causes neuronal degeneration and motor dysfunction is unknown. Based on findings in HD patients and animal models of HD it has been suggested that the genetic defect leads to metabolic compromise with consequent increased sensitivity to glutamate-induced neuronal injury (excitotoxicity). This excitotoxic injury may involve elevated levels of [Ca2+] mitochondrial dysfunction and oxidative stress. Recently, several transgenic mouse models of HD (HDTg) which express the mutant huntingtin gene have been created and exhibit neurologic symptoms and pathology similar to that seen in HD. Recent results from this laboratory and others suggest that HDTg mice do have deficits in metabolic enzyme activity and that neuronal responses to glutamate receptor activation are substantially altered by the expression of the mutant huntingtin protein. We propose to test the hypothesis that mutant huntingtin expression leads to mitochondrial dysfunction and ionotropic glutamate receptor mediated neurotoxicity. This glutamatergic dysfunction may be a consequence of alterations in neuronal metabolism or direct effects on receptor function caused by mutant huntingtin expression. As a model of HD, we will use fluorescence imaging techniques in primary neurons cultured from HDTg mice and their non-transgenic (WT) littermates to test this hypothesis. Our specific aims are to identify the mechanism(s) underlying the potentiation of 1) Glutamate-receptor mediated [Ca2]i responses in HDTg neurons. 2) Glutamate-receptor stimulated mitochondrial depolarization in HDTg neurons. 3) Excitotoxic neuronal death in HDTg neurons. These studies address our long-term goal of evaluating the role of metabolic compromise on glutamate toxicity as it relates to the neuronal dysfunction and death evident in HD. The cell culture models of HD proposed in this work should also provide a convenient means for testing relevant therapies for HD.